Semiconductor silicon carbide (SiC) has considerable bandgap energy in comparison with silicon that is widely used in devices, and is therefore suitable for high-voltage, high-power, and high-temperature operation. There are high expectations for the application of silicon carbide to power devices and other components. The structure of SiC power devices, for which research and development are being actively carried out, can mainly be divided into two classes: MOS devices and junction devices. The present invention relates to a performance improvement in bipolar transistors, which are junction devices.
Following is a reported example of an SiC bipolar transistor.
An example of a typical bipolar transistor has been described in High Power (500 V-70 A) and High Gain (44-47) 4H-SiC Bipolar Junction Transistors (J. Zhang, et al., Materials Science Forum, Vols. 457-460 (2004) pp. 1149-1152). The bipolar transistor described therein is formed by laminating an n−-type high resistance region, a p-type base region, and n+-type emitter regions in the stated order on the surface of a low-resistance n+-type 4H-SiC substrate that is misoriented by 8 degrees to (0001), and the emitter regions are composed of a plurality of long narrow regions. Electrodes are formed in the emitter regions, base region, and collector region to make electrical connections to the exterior.
FIG. 9 shows a cross-sectional schematic view of the bipolar transistor disclosed in the above-described reference. The bipolar transistor 100 is provided with a collector region 101, which is an n-type low-resistance layer; an n-type high resistance region 102; a p-type base region 103; n-type low-resistance emitter regions 104, a p-type low-resistance base contact region 105 formed around the emitter regions; a collector electrode 106; a base electrode 107; emitter electrodes 108; and a surface protective film 109.
FIG. 10 is a drawing that illustrates the typical operation of the bipolar transistor 100. In FIG. 10, the same reference numerals are assigned to the same constituent elements as those shown in FIG. 9. The surface protective film 109 is omitted from FIG. 10 in that the film does not directly relate to the description of the operation.
In the bipolar transistor 100 shown in FIGS. 9 and 10, the main electric current is produced by electrons shown by the arrow 110 that flow from the emitter regions 104 to the collector region 101. The on/off state of the electron current is controlled by a voltage signal applied to the base electrode 107. The direction in which the main electric current flows at this time is the direction facing from the collector region 101 to the emitter regions 104. The bipolar transistor 100 is in an off-state when the voltage between the base electrode 107 and the emitter electrodes 108 is 0 V or less, and changes to an on-state when a positive voltage is applied between the base electrode 107 and the emitter electrodes 108. When the bipolar transistor 100 is in an on-state, the pn junction formed between the base electrode 107 and the emitter electrodes 108 is set with a forward bias, and an electric current based on positive holes flows from the base electrode 107 to the emitter electrodes 108.
A stronger main electric current 110 is preferably controlled with a weaker base electric current in order to operate the bipolar transistor 100 at high efficiency. The current amplification factor (=main electric current/base electric current) is therefore a required parameter. A cause that reduces the current amplification factor is the recombination state on the semiconductor surface such as that schematically shown by the symbol “x,” which is indicated by the reference symbol 111 in the FIG. 10. A large number of surface states caused by uncombined atoms, crystal defects, and the like ordinarily exist on the surface of the semiconductor.
By thermally oxidizing the silicon surface, for example, it is possible to create a silicon/oxide film interface with a low surface-state density that does not negatively affect the device characteristics. On the other hand, it is currently impossible, for example, to sufficiently reduce the surface-state density on the surface of the SiC by using heat oxidation or the subsequently performed heat treatment (POA: Post Oxidation Annealing) and the like. The surface states of the semiconductor surface act as recombination states. For this reason, when the main electric current 110 is ON, electrons 113 injected from the emitter regions 104 and positive holes 112 in the base region 103 coexist in areas of high concentrations of recombination states 111 brought about by the surface states of the surface of the base region 103, as is schematically shown in FIG. 10. The positive holes and electrons (indicated by the arrows 115 and 116) thereby actively recombine, and since reactive base electric current flows without contributing to the operation of the device, the current amplification factor is reduced as a result.
There is a problem in a conventional bipolar transistor 100 in that the positive holes in the base region 103 and the electrons injected from the emitter regions 104 recombine via the surface states of the surface of the base region 103, and the current amplification factor is reduced when a positive voltage is applied between the base electrode and collector electrodes to switch on the device.
There is therefore a need to provide a high-performance bipolar semiconductor device and a manufacturing method thereof which can be applied to a device for controlling the motor of an automobile and to other devices, in which the recombination of positive holes and electrons that is produced via the surface states of the semiconductor surface can be controlled, and in which the current amplification factor is improved.